Current clinical strategies for soft tissue augmentation primarily involve autologous, allogenic, and alloplastic biomaterials. Much of the interest in the autologous approach has focussed on transfer of adipose tissue. This approach is favourable because most patients have expendable reserves of adipose tissue available for transplantation, and the advent of liposuction has facilitated the process of tissue harvesting. However, free fat transfer yields unsatisfactory and unpredictable results, with varying degrees of graft resorption due to a lack of supporting vasculature. In general, only small defects can be corrected with injected autologous fat, and even these limited applications require repeated treatments to maintain the desired volume as the implanted fat is gradually replaced by fibrous tissue interspersed with oil cysts.
Autologous tissue transfer using vascularized flaps that incorporate skin, fat, and muscle requires considerable surgical skill, but can yield far superior results to those obtained with synthetic implants. However, the flaps have high costs in terms of donor site morbidity and deformity, as well as hospitalization and surgical time. The transfer of significant tissue volumes can also result in a weakening of the donor site, particularly in the case of major muscle transfer from the abdominal region. Autologous collagen injectables may be prepared from biopsies or larger samples of the patient's own skin. Such materials are associated with implant resorption and provide only a transient augmentation effect. The amount of tissue that can be obtained using this approach is limited by the creation of a defect in the skin.
Allogenic materials are primarily used as injectable bulking agents for small volume cosmetic applications involving wrinkles and minor defects in the face. The materials temporarily augment the dermal region, smoothing the appearance of the overlying epidermis. In general, these materials have limitations and most require repeated, expensive treatments to maintain the desired effect. Materials such as collagen derived from animal (e.g., bovine, porcine) sources may be associated with hypersensitivity and immune reaction. Rapid implant resorption is also an issue, such that overcorrection (100-200%) at the time of implantation is recommended and repeated treatments (every 3-6 months) are necessary to maintain the desired volume. Cross-linking the collagen with glutaraldehyde may improve resiliency and decrease the immunogenicity of the material. Concerns about disease transmission limit the clinical applicability of xenogenic collagens.
Other allogenic materials such as decellularized human cadaveric dermis that contains collagen, elastin, and glycosaminoglycans (GAGs) may be produced in an injectable form, or in sheets which can be cut to shape for a given application. The materials are non-immunogenic and may last up to two years; however, resorption occurs and overcorrection of up to 200% may be required.
Injectable alloplastic materials, including poly(L-lactic acid) (PLA), have been approved for the treatment of lipoatrophy in patients with HIV. Repeated injections are required and the formation of palpable nodules has been reported. Injectable polytetrafluoroethylene (PTFE) facilitates permanent augmentation without the need for multiple treatments. However, PTFE implants can migrate from the implantation site, have increased risk of infection and have significantly different mechanical properties than soft tissue.
The above autologous, allogenic, and alloplastic approaches to soft tissue augmentation suffer from substantial weaknesses. An alternative approach to soft tissue reconstruction employs a cell-seeded scaffold. Scaffolding materials may be synthetic (e.g., polylactic-co-glycolic acid, polyglycolic acid, polyethylene glycol diacrylate) or naturally-derived (e.g., collagen, fibrin, derivatives of hyaluronan, silk protein). Insofar as naturally-derived scaffolding biomaterials may be preferred, a potential drawback is that they require processing, such as decellularization, prior to use to minimize the potential for immunogenic reaction and to ensure long-term stability.